The present invention relates to a flow device and method, and more particularly to a flow device and method for lowering the exit velocity of a gaseous medium flowing through the device by increasing the divergence of the wall structure at the exit end of the device while preventing unstable flow conditions.
U.S. Pat. No. 3,778,038 granted Dec. 11, 1973, explains a method and apparatus for producing a uniform combustible mixture of air and minute liquid fuel droplets for delivery to the intake manifold of an internal combustion engine. This apparatus includes an intake air zone connected to a variable area throat zone for constricting the flow of air to increase its velocity to sonic. Liquid fuel is introduced into the air stream to minutely divide and uniformly entrain fuel as droplets in the air flowing through the throat zone. Wall structure downstream from the throat zone is arranged to provide a gradually diverging zone for efficiently recovering a substantial portion of the kinetic energy of the high velocity air as static pressure. Such efficient conversion enables the maintenance of sonic velocity air through the throat zone over substantially the entire operating range of the engine to which the air-liquid fuel mixture is supplied.
As further explained in the above U.S. patent, during flow conditions when the pressure ratio across the apparatus is high, a shock occurs downstream from the throat zone. As the pressure ratio increases, the shock moves down the gradually diverging zone and further away from the throat zone. With even higher pressure ratios across the apparatus, the shock moves further down the gradually diverging zone. After shock under any conditions there is a tendency for the flow to separate from the walls of the gradually diverging zone. Usually the flow simply reattaches to the walls but when the shock is far down the gradually diverging zone, there is not sufficient time for such reattachment and an excessively high velocity jet is formed.
The above U.S. patent also discloses that the device functions to control the mass flow of air being supplied to the engine since the air flow is maintained at sonic velocity through the throat zone over a wide range of engine conditions. Hence, under unvarying atmospheric conditions the mass flow rate of air being supplied to the engine is directly proportional to the cross-sectional area of the throat zone. Finally, as is apparent from the above U.S. patent, the liquid delivery means may be eliminated and the device used solely as a mass flow control for air or any gaseous medium.
The particular divergence of the wall structure downstream from the throat zone is extremely important in order to efficiently recover the kinetic energy of the high velocity mass as static pressure. As explained above, such efficient energy recovery enables sonic velocity at the throat zone over a wide range of downstream pressure conditions. However, the gradually diverging zone formed by the wall structure may be such that the exit velocity of the mass is excessive under the conditions mentioned above when the pressure ratio across the apparatus is high thereby causing a shock to occur far down the gradually diverging zone. Then the flow does not reattach to the walls and a high velocity jet emerges from the apparatus. In a carburetor application, for example, excessive exit velocity causes the air-fuel mixture to impinge upon the manifold floor which prevents the mixture from being delivered to the cylinders of the engine in a homogeneous state.
Utilization of a larger angle of divergence of the wall structure downstream from the throat zone causes the shock to occur much closer to the throat zone which allows reattachment of the flow to the walls before it emerges from the apparatus. The exit velocity of the mass is thereby lowered. However, such wider angles in the diverging zone may result in unstable flow at the lower end portion of that zone, particularly under conditions when the angle of divergence is quite wide and the throughput is low. Also, switching of the mass from one side to the other in the low end portion of the diverging zone may occur under the same conditions. Lower efficiency often results since some of the kinetic energy of the high velocity mass is lost to turbulence at the lower end portion of the diverging zone. Switching causes droplet conglomeration of the fuel and poor charge distribution of the mass delivered to the manifold.